The present invention relates to traveling-wave tubes and particularly to TWTs having helix interaction structure placed coaxially within a conductive tubular envelope, the function of which is to shield the electromagnetic wave of the signal. A proper arrangement of metallic conductive fins or elements is provided so that the shielding may be selective in order that the helix dispersions are almost null or even negative and a remarkable extent of the operative frequency band may be attained with a substantially constant phase velocity. Remarkable advantages then result from the use of electron TWTs whose characteristics are in accordance with the present invention.
It is well known by the skilled in the art that in a traveling-wave tube to be used as a tube amplifier, the phenomenon of interaction is utilized, which may be provided between an electron beam that after being emitted by an electron gun cathode enters a delay line, or interaction structure, and moves through the same, and the electromagnetic-wave of the signal which propagates along a considerable length of the electron tube. The propagation of the traveling wave is in the same direction of electron motion. In order that the interaction phenomenon may take place a well predetermined velocity ratio is however necessary, namely between electron velocity of said electron beam and propagation velocity of the electromagnetic wave phase in the structure.
Furthermore, in order that this phenomenon of interaction may be really useful for practical purposes as desired, it is also necessary that a right velocity ratio may be maintained on all frequencies of the operative band, while such a velocity ratio is generally compromised because the propagation velocity of electromagnetic wave phase of the signal in the structure changes on the basis of the frequency. More than that, it must be added that, because of such variation of wave phase velocity in the structure, which is defined as dispersion of the phase velocity, it is quite possible that the condition giving rise to the interaction phenomenon is lost when the bandwidth exceeds a certain value.
The delay lines being used as interaction structures in traveling-wave tubes and forming transmission lines having very small dispersions and a very small distorsion with time delay, may be of many types, each one having characteristics of phase velocity dispersion more or less accentuated. One of the simpler, better and more used is the helix type as a slow wave circuit where the waves propagate along the wire almost at the light velocity, the phase velocity in the axial direction being then nearly equal to the velocity of light multiplied by the pitch/circle ratio.
Helixes are then delay lines having a lower dispersion, the usually attained bandwith by traveling-wave tubes wherein they are used being one octave. As regards broader bandwith, however, the dispersion of phase velocity provides some fall of output useful power near the edges thereof, which in many cases cannot be borne.
In electron tubes wherein for the purposes of the present invention a helix type interaction structure is provided as evidenced above, the propagation velocity of the wave phase increases when the frequency is decreasing (i.e. positive dispersion), while the phase velocity should have a lightly increasing state by a frequency increase (i.e. negative dispersion), or at the most a null dispersion to maintain the most suitable interaction conditions for said purposes.